None.
The present invention is a method for preparing thin lead samples to test for emission levels of naturally occurring radioactive contaminants, such as polonium.
Lead has a variety of uses in the electronics industry, and is commonly used to make electrical contacts in high density high memory computer chips. In particular, lead is a major component in the solder used to form many of the connections and contacts required in computer chips and circuits. These solder connections are typically in the form of small spheres, or solder bumps, approximately two thousands of an inch in diameter.
Memory chips are based on binary logic and typically consist of small solid state capacitors with potential wells that are either filled with electrons (0) or not filled (1). A xe2x80x9csoft errorxe2x80x9d is said to occur when an empty well (1) is changed to a filled well (0). Such soft errors may occur when ionizing radiation from the lead solder passes through the substrate of the chip, which is often formed of silicon, and releases electrons from covalent bonds. These free electrons collect under the positive electrode, and thereby fill the normally empty potential well. A main source of the ionizing radiation which can cause soft errors is alpha particle emission from polonium-210, a radioactive daughter of lead-210 and a contaminant in commercial lead.
Such soft errors, though not permanent, greatly affect the accuracy and reliability of the chip. In addition, as dimensions and operating voltages of computer chips are reduced to satisfy the demand for higher density and speed, their sensitivity to radiation increases. Thus, it is imperative that clean or xe2x80x9clow-alpha leadxe2x80x9d (LAL) be used in solder connections made on computer chips and circuits. As a result, it is necessary to test lead for the levels of alpha particle emission to determine whether the lead is suitable for use in electronic and computer applications.
Current methods of determining the polonium impurities in commercially available lead are time consuming, difficult, and expensive. One method involves dissolving a representative sample of the lead in acid and plating out the polonium on a metal disk. Such a method is undesirable because it is destructive, time consuming, and requires the use of caustic chemicals. Another method involves testing the polonium concentration in lead samples using a large area alpha particle detector to test a large area sample of lead. Unfortunately, the alpha particle flux only reflects the polonium impurities in the upper portion of the sample closest to the sample surface. As a result, the polonium concentration throughout the sample must be estimated based on the alpha particles detected from the surface. This can lead to an imprecise determination of polonium concentration, and in turn can lead to an imprecise prediction of alpha particle emission.
Thus, there is a need in the art for a method of preparing and testing a lead sample to determine a level of alpha particle emission which is rapid, accurate, and economical.
The present invention relates to an improved method of preparing samples of lead for testing to determine an amount of alpha particle emission caused by naturally occurring radioactive contaminants in the lead, such as polonium. The method involves first obtaining a very thin slice from a lead sample. Next, the slices are flattened to remove any wrinkles caused by obtaining the slice, and possibly to increase the surface area of the sample slice. The flattened slices are then heated to a temperature that facilitates polonium percolation to the top and bottom surfaces of the sample slices. Finally, the lead slices are tested to determine an amount of alpha particles emitted from the slices.